1. Field of the Invention
This invention relates to an improvement in a high pressure discharge lamp apparatus such as a high pressure mercury lamp apparatus, a high pressure sodium lamp apparatus, or a high pressure metal-halide lamp apparatus. The present invention especially concerns an improvement in a high pressure discharge lamp apparatus of the type comprising a high pressure discharge tube, a current limiting device, for instance a choke coil, and a pulse generator output terminals of which are connected in parallel to the electrodes of the discharge tube for lighting the tube with a lamp voltage nearly equal to a power source voltage.
2. Description of the Prior Art
Generally, in a conventional high pressure discharge lamp apparatus, a current limiting device such as a choke coil and a discharge tube are connected in series across a power source.
In the abovementioned type of the high pressure discharge lamp apparatus, a voltage of the power source should be maintained over 1.5 times a voltage of the discharge tube for preventing an extinction of ignition in the tube. Therefore a voltage drop across the current limiting device is high, and a power loss at the current limiting device is rather large. Accordingly, a dimension and a weight of the current limiting device become undesirably large.
For improving the high pressure discharge lamp apparatus to eliminate the abovementioned defects, a pulse generator may be provided in a manner to give pulses to the discharge tube. By such impression of pulses across the discharge tube, the lamp voltage can be selected so high as to be nearly equal to the power source voltage.
The conventional discharge lamp apparatus having a pulse generator as abovementioned still has the following problems:
(1) In a case that the high pressure discharge lamp is ignited in every cycle of the power source voltage by impressing of pulse by the pulse generator, a lamp voltage can be selected so high as to be nearly equal to the power source voltage, and an impedance of the current limiting device can be selected small. But, when the discharge tube starts to ignite, a lamp current grows excessively large, and therefore the impedance of the discharge tube such as the choke coil becomes almost zero for a magnetic saturation due to the overcurrent. Therefore, for avoiding the magnetic saturation, the choke coil as the current limiting device should be designed so as to have relatively large sectional area of a core thereof, thereby making the current limiting device large in dimension and expensive in cost.
(2) In order to solve the abovementioned problem, another conventional high pressure discharge lamp apparatus has been devised as shown in FIG. 1, wherein a current limiting device 2 such as a choke coil, an auxiliary current limiting device 5 and a discharge tube 3 are connected in series across a power source 1. The auxiliary current limiting device 5 of a relatively large size has more large capacity of current limitation than that of the current limiting device 2. And a semiconductor switching device 6 is connected in parallel to the auxiliary current limiting device 5. A pulse generator 4 is connected in parallel to the discharge tube 3 for supplying reignition pulses to the discharge tube 3.
In the abovementioned high pressure discharge lamp apparatus, a duty time of the current limiting device 5 is controlled by firing angle controlling action of the semiconductor switching device 6 in such a manner that at the starting the duty time of the switching device 6 is very small and at the steady lighting state the duty time is very large, so that effectively the auxiliary current limiting device 5 is substantially short circuited by the switching device 6 at a steady lighting state of the lamp. Therefore a lamp current may be maintained for the same degree between at a steady lighting state and at a starting transient state.
Generally, in the high pressure discharge tube an ignition of the discharge tube extincts every period when the lamp current is zero. Accordingly, if a series insertion of a thyristor to the high pressure discharge lamp of conventional type for attempting to limit the lamp current by changing its firing angle, the tube does not work due to zero current period in every cycle. Therefore, in the lamp apparatus of FIG. 1, lamp current is always supplied from the power source 1 through the auxiliary current limiting device 5, which is connected in parallel to the thyristor or the like current limiting device 6, thereby to allow the current to flow without zero period made by the thyristor 6. However the problem of the conventional lamp apparatus is that though the current limiting device 2 can be designed relatively small, a large auxiliary current limiting device 5 has been necessary. Accordingly the lamp apparatus can not be made in small size.